I've skimmed the 'net every few years for this one to no avail, I figure it may strike a chord with someone. :-)

There was a VERY short (but dense) Atari Basic demo that took hours to run. When it completed, it would show, drawn in graphics mode 8+16, from a three-quarters perspective, a 3D surface, with hidden lines
removed. The final image looked rather like a hat.

I seem to remember it being referred to as "Archimedes Spiral" in the demo.

I recall the article with the program listing had some suggested modifications (disabling hidden line removal made the program run faster) -- I remember the bracketed editor's quote "[It still looks
like a hat to me --Ed.]".

I grew up in Michigan, this could conceivably have been from the M.A.C.E. Journal newsletter...

Allen MacroWare's "Printwiz" used the image as their title screen ( http://hobart.googlepages.com/printwiz.jpg )